Signal interface circuit

ABSTRACT

Interface unit for voltage input signals comprising two or more input channels. The input signals of these two or more input channels are connected alternately by an analog multiplexer to an analog-to-digital converter. The A/D converter comprises an integrated sigma-delta modulator circuit which generates a digitized 1-bit signal representing the input signal voltage level for a control unit irrespective of whether the input channel signal is digital or analog. By means of the invention all input voltage channels are made similar such that the input channels of the interface unit can receive an analog or digital signal irrespective of each other.

BACKGROUND OF THE INVENTION

The invention relates to a signal interface for a voltage input signal.

In industrial instrumentation, industrial electric drives, such as motordrives and frequency converters, and also in other drives there is oftena need for analog or digital signals that signal various quantitiesrelating to the operation of the drive. Such signals may include, forexample, control signals and measurement signals. Signal sources mayinclude a rotatory switch or a push button, a guard switch, sensors etc.The voltage level and voltage type depend on the signal source but varytypically from DC voltage of 0 to 10 VDC to alternating voltage (AC) of230 VAC of different switch signals.

Conventionally, analog and digital signals have been separated byforming them already on the hardware level as separate groups of inputchannels in such a way that the analog channels are converted intodigital form with multiple-input SAR (Successive Approximation Register)type A/D converters, and digital inputs typically utilize optoisolatorsto provide galvanic isolation from the common potential of the channels.To improve resolution, the measurement range of the analog inputs istypically dimensioned to ten volts, whereby special measures have had tobe used for analog voltages higher than this. Correspondingly, digitalchannels have typically been intended for one single nominal voltagelevel, i.e. either for a DC level of 24 volts or in special cases for anAC level of 230 volts.

In digitizing, a converter of the SAR type has, however, severaldisadvantages, a few of which are presented in the following. 1) Even ashort interference peak affecting an analog signal to be measured at thesampling moment may lead to a greatly erroneous digitized value becausethe sampling may last only a few nanoseconds. 2) To prevent interferencethe measuring circuit must be provided with hardware filters, and on thedigital signal processing side, digital filters and discriminators ofdifferent levels must still be arranged. 3) Conventionally, the analogsignals to be digitized are bound to the same ground potential, wherebyloop currents are easily generated and may cause mains-frequencycommon-mode interference that is difficult to prevent. 4) Fast SARconverters capable of over 12-bit conversion are expensive, so the inputchannels must be adapted for operation in a given limited voltage rangein order for the resolution of the digitization to be sufficient. 5)Hardware filtering provided due to interference usually preventsmeasurement of digital input signals by means of a high-speed SARconverter.

U.S. Pat. No. 4,316,132, U.S. Pat. No. 5,349,351 and U.S. Pat. No.7,126,514 disclose multiplexing of analog control signals to one or moreA/D converters.

For digital input signals, in turn, the galvanic isolation hasconventionally been provided by using optoisolators, which enable safedetection of even 230-volt voltage levels. Optoisolators have, however,several disadvatages, a few of which are presented in the following: 1)the relatively high current level required by LED on the input side (5 .. . 10 mA), which results in either very limited input voltage ranges orin a complex and expensive constant-current generator; 2) this resultsin loading of a signal source, which is not always acceptable; 3) agreat power loss at a high input voltage; and 4) a need for separaterectification when alternating voltages are indicated, and also for acomparator if it is desirable to transfer polarity information.

To keep the costs at a reasonable level, the input channels havetypically been positioned, according to standard procedure, as part of aparticular motor control board, which is often positioned close to thepower stage of the frequency converter or even constructed as part ofit. This involves several disadvantages.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a signal interface circuitof a new type. The object of the invention is achieved with a signalinterface unit and an electric motor drive that are characterized bywhat is stated in the independent claims. Preferred embodiments of theinvention are described in the dependent claims.

The interface unit for voltage input signals comprises two or more inputchannels, to each of which a digital or analog input signal can beconnected irrespective of other input channels. The input signals ofthese two or more input channels are connected alternately by an analogmultiplexer to an analog-to-digital (A/D) converter. The A/D convertercomprises an integrated sigma-delta modulator circuit which generates adigitized 1-bit signal representing the input signal voltage level to beinterpreted by a control unit irrespective of whether the input channelsignal is a digital or analog signal. By means of the invention allinput voltage channels are made similar irrespective of whether thesignals are analog or digital. The input channels of the interface unitaccording to the invention may receive an analog or digital signalirrespective of each other, in other words the same interface unitstructure may receive a desired number of both analog and digital inputsignals, only analog input signals or only digital input signals. Theinvention makes it possible to avoid the need for different units fordigital and analog signals as well as the need for two units if bothdigital and analog signals are to be processed. This results in costsavings in both manufacturing and installing. Further, an input signalmay be flexibly converted from a digital signal to an analog one or viceversa during the use. The nominal measurement range of the inputchannels can be selected freely, for example by changing the values ofthe voltage divider resistors at the inputs, so the measurement range ofmains-voltage level signals, for example, may be ±500 volts, whereas theinput voltage range of a channel intended for measuring a setpoint valuemay be selected to be for example ±12 volts, if desired.

In accordance with an embodiment of the invention, the integratedgalvanic isolation is integrated into a sigma-delta modulator circuitbetween the analog input and the digital output. Further, the interfaceunit may be provided with an integrated, galvanically isolating circuithaving at least one control signal input from a control unit and atleast one control signal output which has a galvanic isolation from thecontrol signal input and provides either directly or indirectly acontrol signal for the analog multiplexer. In this way, the commonpotential level of the galvanically isolated input signal groups isfreely selectable, whereby interference-generating ground currents orother loop currents are not generated. In an embodiment of theinvention, this integrated, galvanically isolating circuit comprises aDC-DC converter circuit having at least one control signal input fromthe control unit, and an operating voltage input, as well as at leastone control signal output and an operating voltage output which aregalvanically isolated from the inputs, whereby said control signaloutput provides said control signal directly or indirectly, and saidoperating voltage output generates an operating voltage for the analogmultiplexer and the analog side of the sigma-delta modulator circuit.Owing to this galvanically isolated operating voltage, no separateauxiliary voltage supply is needed, which reduces the manufacturingcosts and improves galvanic isolation.

Electric motor drives are a particular application area of theinvention. In an electric motor drive, the interface unit according tothe invention is preferably positioned on a separate circuit board,which is installed in a board connector on the main circuit board of themotor drive, and the control unit receiving a digitized output signaland generating control of the multiplexer is positioned on the maincircuit board. This also includes an embodiment in which the interfaceunit is connected with a (serial communication) cable to the motorcontrol board, on which the control unit is positioned. By means ofthese solutions, structural problems related to conventional I/Oconnections can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in connection withexemplary embodiments, referring to the attached drawing, in which FIG.1 shows a circuit diagram of an exemplary circuitry applying theprinciples of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

In the example shown in FIG. 1, the invention is implemented with twoseparate modules (e.g. circuit boards) 20 and 30, but the invention maybe implemented as one or more modules as well.

The example circuitry of FIG. 1 has seven input channels I₁-I₇, each ofwhich is connected to the corresponding input terminals (17)-(11) of theanalog multiplexer 204 via voltage dividers formed by resistors R1, R2,R3 and R4. It is to be noted, however, that the number N of inputchannels may be any N≧2. The number of input channels in the measurementmay be increased from the seven channels shown in FIG. 1 by means of amultiplexer circuit of a different type or by using several multiplexercircuits. The example circuitry utilizes a multiplexing circuit CD4051manufactured by National Semiconductors Inc, for example, but alsovarious analog multiplexers implemented with the CMOS technology areavailable with power consumption which is typically only in the range ofmicrowatts. The common “zero” potential level V_(com) of the inputchannels provides a galvanically isolated, floating measurementpotential level that can be set freely, whereby nointerference-generating ground or other loop currents are generated. Theresistive voltage divider R1-R2-R3-R4 attenuates the input voltages ofthe channels I₁-I₇. The nominal measurement range of the input channelsmay be selected freely by changing the resistor values in the voltagedivider R1-R2-R3-R4, so that measurement range of mains-voltage levelsignals may be ±500 volts, whereas the input voltage range of a channelintended for measuring a setpoint value may be selected to be ±12 volts,for example. Since the multiplexer CD4051 cannot deliver negativevoltage levels, a level shifting voltage of +2.5 volts has been selectedto serve as a common point to the input channels in the exemplarycircuitry. This level shifting voltage may be achieved with a zenerdiode D1 or a corresponding reference circuit. Owing to voltage divisionand level shifting, even all negative input voltages are delivered aspositive signals to the output of the multiplexer 204 and further to bedigitized.

The input terminals 10 to 17 of the multiplexer 204 are selected to beactive and connected alternately to an output Q by providing the desiredbinary information to the address inputs A0 to A2, for example from abinary counter 202, which is, in turn, clocked and reset via a digitalisolator 200 from a control module 30 controlling the whole of themeasurement, for instance with a microprocessor.

Since the resistance of the CMOS switches of the multiplexer 204 is inthe exemplary circuitry relatively high and the resistance value variesfrom one switch to another, the analog signal obtained from the Q outputof the multiplexer 204 may not be applied as such to the voltage dividerformed by resistors R5 (12 kΩ) and R6 (860Ω), but a separate bufferamplifier 205 may be used as an impedance converter with unity nominalvoltage amplification. The buffer amplifier may be implemented with asuitable integrated operational amplifier, for instance.

In the example of FIG. 1, the digitizing is performed by a Σ/Δ modulatorhaving excellent absolute accuracy and stability while still beinginexpensive. Owing to its principle of operation, the Σ/Δ modulator isalso particularly resistant to different interference peaks. In thepreferred embodiment of the invention shown in FIG. 1, a measuring anddigitizing unit 206 is implemented with an integrated Σ/Δ modulatorcircuit AD7401 manufactured by Analog Devices. AD7401 is a second-orderΣ/Δ modulator, which converts an analog input signal into a high-rate1-bit data stream and further comprises digital isolation implementedwithin an integrated circuit chip. This isolation is illustrated with abroken line within the unit 206 in FIG. 1. Owing to this, the measuringand digitizing unit 206 also functions as a galvanically isolatinginterface between the analog part 20B and the digital part 20A of themodule 20. Alternatively, if for instance an AD converter with nointernal galvanic isolation is used as the measuring and digitizing unit206, galvanic isolation may be implemented with a separate isolatorcircuit, such as AduM5240 used as the isolator circuit 200. The analogmodulator of the integrated circuit AD7401 continuously samples a node28 in the voltage divider R5-R6, whereby no external sample and holdcircuitry is needed. In connection with some AD converter circuits,however, an external sample and hold circuit or another correspondingmeasuring circuit may be required. In order to function correctly, theΣ/Δ modulator AD7401 needs a clock signal Clock1 of 20 MHz at the most,whereby a one-bit data stream DATA whose pulse width has been modulatedin one way is supplied, in phase with the clock signal Clock 1, to thecontrol module 30, where it can be subjected to digital filtering anddecimation 300 to produce multibit measuring information. A Sinc³filter, for example, is preferable because it is one order higher thanthe second-order modulator AD7401. If, in addition, a decimation ratio256 is used, the result is a 16-bit word at a rate of 62.5 kHz when theexternal clock signal Clock1 fed to the unit 206 is 16 MHz. When 1 bitis used for the sign, a resolution of ±15 bits is obtained. The digitalfiltering and modulation 300 may be implemented with an integratedcircuit or a signal processor. In the example of FIG. 1, digitalfiltering and decimation are positioned in a different processing/logicunit 30, such as in an electric motor drive, because this arrangementprovides several advantages. The module generating an analog outputsignal may be implemented in smaller size. Fewer signal wires are neededbetween the module 20 and the processing/logic unit because themeasuring data is transferred as 1-bit data. The digital filtering anddecimation 300 may, if desired, be implemented by way of program in theprocessor of the processing/logic unit 30.

The control signals of the module 30, i.e. a clock signal Clock2 and areset signal Reset, are connected to a dual-channel digital isolator 200that isolates the digital I/O part 20A of the module 20 from thefloating part 20B. In the exemplary embodiment, the digital isolator 200is implemented with an integrated circuit comprising an integrated DC/DCconverter, such as AduM5240 manufactured by Analog Devices Inc. Thedigital isolator's 200, such as AduM5240's, own internal, galvanicallyisolating current source preferably generates auxiliary power requiredin galvanically isolated and floating measuring potential for all othercomponents of the floating module part 20B, whereby no separateauxiliary voltage source is required. In the example of FIG. 1, thetotal current consumption of the circuits 200, 202, 204, 205 and 206 onthe side of the measuring potential is about 10 milliamperes, which thecircuit AduM5240 is capable of generating as its nominal output level.

The operating voltage +5V and zero potential (ground) Gnd of the inputside of the digital isolator 200, which is positioned in thenon-floating module part 20A, are preferably supplied from the module30. Correspondingly, also the operating voltage +5V and zero potential(ground) Gnd of the digital side of the sigma-delta modulator 206 arepreferably supplied from the module 30.

The total costs of the components in the measuring circuit according tothe embodiment shown are minimized but the input voltages are stillmeasured as accurately as possible with a sigma-delta modulator capableof 16-bit resolution.

The whole measuring process is typically controlled from a control unit302, such as a microprocessor, which may be positioned on a separatemodule 30, for example in connection with an electric drive. Themeasurements are preferably carried out at a speed selected byprogramming, whereby the measuring result of the analog signals may, ifrequired, be averaged out to minimize the effect of interference andincidental errors. Correspondingly, measurement of accurate signalamplitude is not the intention with digital-type inputs but what isimportant is the moment of time when a change in the state informationtakes place.

With analog inputs, 16-bit resolution easily results in a speed at whicheach of the seven input channels are measured for instance a thousandtimes a second. If the channels are allocated to be digital, the speedmay be tenfolded by reducing the resolution to 10 bits, for example.Having one channel, the circuitry is capable of detecting a change inthe state information, for example, within a few microseconds, which issufficient for most of the digital signals.

Next, let us consider the measuring process. The measuring cycle isstarted every time by giving a reset pulse from the control unit 302along a Reset line. A reset pulse is transferred through the digitalisolator 200 to the input terminal R of the binary counter 202, wherebythe output of the counter is reset to the zero state. Thus, themultiplexer 204 selects the input channel I0, in other words themeasuring cycle is started by measuring a level shifting voltage of+2.500 volts via the input I0 of the multiplexer. At terminal I0 thereprevails 2.500 V−((2.500 V/994700Ω)*4700Ω)=2.500 V−0.0118 V=2.4882 V,which appears at the input terminal 28 of the Σ/Δ modulator 206 as 1/15,i.e. 0.1659 volts.

The control 302 further supplies the clock signal Clock2 via the digitalisolator 200 to the clock input terminal of the binary counter 202,whereby the output value Q2Q1Q0 of the binary counter 202 increases insynchronism with the clock signal, selecting alternately one of theinputs 11 to 17 and one of the outputs I₁-I₇ for measurement.

If it is assumed that the highest negative input voltage of a giveninput channel I₁-I₇ is −500 V, the overall voltage of −502.5 voltsaffects over the input voltage divider R1-R2-R3-R4, of which voltage−2.374 volts remains over the resistor R4. The voltage of thecorresponding input channel 11-17 of the multiplexer 204 is thus 2.500V+(−2.374 V)=0.1257 V. If the highest positive input voltage of a giveninput channel I₁-I₇ is +500 V, the voltage of the corresponding inputchannel 11-17 of the multiplexer 204 is 4.8510 V. When these voltagesare further attenuated to 1/15, voltages of 0.0084 V and 0.3234 V appearat the input terminal 28 of the Σ/Δ modulator 206. From these values,the voltage measured by the level-shifting channel is yet to besubtracted, so that the final measuring results are 0.0084 V−0.1659V=−0.1575 V, and 0.3234 V−0.1659 V=+0.1575 V. The values are naturallyequal but opposite in sign. When the overall attenuation of the voltagedividers is 211.64*15=3164.6, it is easy to perform multiplication3164.6*0.1575 V=500 V for a check. When a voltage of +500 volts ismeasured, the input range of the Σ/Δ modulator is slightly exceeded, sothe real measuring range in the positive direction is 0.3200 V/0.3234V*500 V=494.7 V.

The present invention is not intended to be limited to the abovecomponents, component values or circuit solutions but it is obvious thatby changing component values, components and circuit solutions,properties of the device may be changed without deviating from the basicprinciples of the present invention.

In an embodiment of the invention, the module 20 of FIG. 1 isimplemented as a small circuit board module that may comprise a desirednumber of input channels. The circuit board 20 of the module is providedwith a suitable connector, with which the module circuit board 20 may bepositioned in the corresponding connector on the motherboard.Preferably, the motherboard also comprises the functions of the module30. With this structure, several advantages are achieved compared withconventional solutions, in which the input channels with theirconnectors are typically positioned for example as part of a frequencyconverter's motor control board, which is often positioned close to thepower stage of the frequency converter or which is even constructed aspart of it. This has resulted in numerous disadvantages. Due to itscomplexity, the frequency converter board, such as a motor controlboard, has usually at least six layers, whereby positioning simple I/Ofunctions on the mother board means wasting expensive area. If the inputchannels were made directly to the motor control board at the installingstage, there would be, due to the bending caused by mechanical forces inthe installing, a risk of the numerous and partly large-sizesurface-mounting components being damaged, which would not be revealeduntil at the start-up stage. Maintenance is expensive and may causedelays. A natural route must be reserved for all I/O cables, which isoften very difficult due to the narrow space available for the frequencyconverter board or motor control board and leads to solutions wheremaintenance measures are substantially complicated, and not even thelevel of electromagnetic interference (EMC) achieved is sufficient.Further, it is often preferable, for cabling, connecting and maintenancepurposes, to position the I/O interfaces of several drive groupscentrally in one single place completely separately from the motorcontrol, but it cannot be done because the I/O functions are positioneddirectly on the motherboard. By means of the module board 20 accordingto a preferred embodiment of the invention, which is positioned with aconnector on the motherboard on the motor control board of a motor driveor connected with a serial-communication cable to the motor controlboard, on which the control unit is positioned, above prior art problemscan be avoided and desired advantages described above can be achieved.

It will be obvious to a person skilled in the art that as the technologyadvances, the basic idea of the invention may be implemented in aplurality of ways. The invention and its embodiments are thus notlimited to the above examples but may vary within the scope of theclaims.

1. An interface unit for voltage input signals, comprising: ananalog-digital converter for converting an input signal voltage into adigital output signal for a control unit, the input signal voltage beinggalvanically isolated from the digital output signal; two or more inputchannels; and an analog multiplexer configured to switch the inputsignal of said two or more input channels alternately to saidanalog-digital converter; wherein each input channel is configurable toreceive a digital input signal instead of an analog input signal whileone or more of other input channels is configured to receive an analoginput signal; and wherein said analog-digital converter comprises anintegrated sigma-delta modulator configured to generate a digitized1-bit signal representing both a voltage level of the digital inputsignal or signals and the analog input signal or signals forinterpretation of the control unit.
 2. An interface unit according toclaim 1, comprising by galvanic isolation integrated into saidsigma-delta modulator circuit between the analog input and the digitaloutput; an integrated galvanic isolating circuit having at least onecontrol signal input from said control unit and at least one controlsignal output isolated galvanically from the control signal input, whichcontrol signal output provides either directly or indirectly a controlsignal for the analog multiplexer.
 3. An interface unit according toclaim 2, wherein said integrated galvanically isolating circuitcomprises a DC-DC converter circuit having at least one control signalinput from the control unit, and an operating voltage input, as well asat least one control signal output and operating signal outputgalvanically isolated from the inputs, whereby said control signaloutput provides directly or indirectly said control signal, and saidoperating voltage output provides an operating voltage for the analogmultiplexer and for the analog side of the sigma-delta modulatorcircuit.
 4. An interface unit according to claim 2, comprising a binarycounter configured to generate, clocked by said control signal output, acyclic binary number sequence to serve as the control signal of theanalog multiplexer.
 5. An interface unit according to claim 1,comprising digital means for filtering and decimating a digitized 1-bitsignal generated by the sigma-delta modulator, so that a multi-bit,digitized, actual current or voltage value is obtained for the controlunit.
 6. An interface unit for voltage input signals, comprising: ananalog-digital converter for converting an input signal voltage into adigital output signal for a control unit; two or more input channels;and an analog multiplexer configured to switch the input signal of saidtwo or more input channels alternately to said analog-digital converter,wherein one input channel of the analog multiplexer is connected to aconstant voltage for calibration of the interface units; wherein eachinput channel is configurable to receive a digital or analog inputsignal irrespective of other input channels; and wherein saidanalog-digital converter comprises an integrated sigma-delta modulatorconfigured to generate a digitized 1-bit signal representing a voltagelevel of the input signal for interpretation of the control unitirrespective of whether the signal of the input channel is digital oranalog.
 7. An electric motor drive comprising an interface unit, saidinterface unit comprising: an analog-digital converter for converting aninput signal voltage into a digital output signal for a control unit,the input signal voltage being galvanically isolated from the digitaloutput signal; two or more input channels; and an analog multiplexerconfigured to switch the signal of said two or more input channelsalternately to said analog-digital converter; wherein each input channelis configurable to receive a digital input signal instead of an analoginput signal while one or more of other input channels is configured toreceive an analog input signal; and wherein said analog-digitalconverter comprises an integrated sigma-delta modulator configured togenerate a digitized 1-bit signal representing both a voltage level ofthe digital input signal or signal and the analog input signal orsignals for interpretation of the control unit.
 8. An electric motordrive according to claim 7, wherein said control unit comprises acontrol processor or logic of the electric motor drive.
 9. An electricmotor drive according to claim 7, wherein the interface unit ispositioned on a separate circuit board, which is connected with aserial-communication cable to a motor control board of the motor driveor installed in a board connector on the motor control board of themotor drive, and wherein the control unit is positioned on the motorcontrol board.
 10. An electric motor drive according to claim 7,comprising: an integrated galvanic isolating circuit integrated intosaid sigma-delta modulator between the analog input and the digitaloutput, the integrated galvanic isolating circuit having at least onecontrol signal input from said control unit and at least one controlsignal output isolated galvanically from the control signal input, whichcontrol signal output provides a control signal for the analogmultiplexer.
 11. An electric motor drive according to claim 10, whereinsaid integrated galvanic isolating circuit comprises: a DC-DC convertercircuit having at least one control signal input from the control unit;an operating voltage input; and at least one control signal output andoperating signal output galvanically isolated from the control signaland operating voltage inputs, wherein said control signal outputprovides directly or indirectly said control signal, and said operatingvoltage output provides an operating voltage for the analog multiplexerand for the analog side of the sigma-delta modulator circuit.
 12. Anelectric motor drive according to claim 10, comprising: a binary counterconfigured to generate, clocked by said control signal output, a cyclicbinary number sequence to serve as the control signal of the analogmultiplexer.
 13. An electric motor drive according to claim 7,comprising: digital means for filtering and decimating a digitized 1-bitsignal generated by the sigma-delta modulator, to provide a multi-bit,digitized, actual current or voltage value for the control unit.
 14. Aninterface unit according to claim 7, wherein an input channel of theanalog multiplexer is connected to a constant voltage for calibration ofthe interface unit.